III-V and II-VI compound semiconductors with variable band gaps (Eg) and lattice constants are needed for numerous electronic and optoelectronic applications, including light emitting diodes, laser diodes, photodetectors, solar and photovoltaic cells, high speed switches, and the like. Directional solidification from the melt is by far the fastest, cheapest, most reliable, and, therefore, the preferred method for producing large scale, device grade, single crystal substrates. Unfortunately, only binary compounds (like GaAs, GaSb, and InP) have been successfully commercially produced in large quantities from melts having discrete energy band gaps and lattice constants. In principle, the band gap and the lattice constant can be tuned in ternary, quaternary, or higher order systems by adjusting the composition of the substitutional cations and anions. However, in practice melt-grown ternary and higher order compounds are compositionally inhomogeneous (see, e.g., Bachmann et al., “Melt and Solution Growth of Bulk Single Crystals of Quaternary III-V Alloys”, Progress in Crystal Growth and Characterization, 2(3):171-206 (1979)) and exhibit high density of defects, such as cracks, inclusions, precipitates, dendrites, and dislocations. These defects are due to several reasons, including large lattice mismatch between the constituent binaries, wide separation between the liquidus and solidus curves in the pseudo-binary phase diagrams, differences in thermal expansion coefficients of the binary compounds, and miscibility gaps.
FIG. 1 illustrates a conventional horizontal Bridgeman apparatus 1 for growing a binary semiconductor single crystal boule. In this method, a crucible 3 containing a crystal growth seed 4 is pulled through a wide hot zone of a furnace having heater coils 2 or other heating elements. The semiconductor material contacting the crystal growth seed and which has already passed through the hot zone of the heater coils is solid. The remainder of the semiconductor material 5 located in the hot zone of the furnace is in the liquid state. Thus, the temperature in the hot zone of the furnace is maintained above the liquidus temperature of the binary semiconductor material. The temperature versus location plot in FIG. 1 shows that the temperature in the hot zone is sufficient to maintain the semiconductor material in the hot zone in the liquid state.
FIG. 2 illustrates a conventional horizontal Floating Zone apparatus for growing a binary semiconductor single crystal boule. In this method, a crucible 3 containing a crystal growth seed 4 is pulled through a narrow hot zone of a furnace having heater coils 2 or other heating elements. The semiconductor material contacting the crystal growth seed and which has already passed through the hot zone of the heater coils is a solid single crystal. The narrow portion 5 of the semiconductor material located in the hot zone of the furnace is in the liquid state. The tail portion 6 of the polycrystalline semiconductor material that has not yet passed through the hot zone is in the solid state. The temperature versus location plot in FIG. 2 shows that the temperature in the tail portion 6 behind the hot zone 2 is maintained below the solidus temperature of the semiconductor material to keep the semiconductor material in the tail portion in the solid state.
Ternary and quaternary semiconductor materials are currently produced in the form of thin layers by non-equilibrium growth techniques (from diluted solutions and vapor phase) on binary substrates using buffer layers to relieve misfit related stresses at the epilayer-substrate interface. One disadvantage of epitaxial technology is its high cost. In addition, the buffer layer technology is not optimized for all systems, and, often devices exhibit large leakage currents due to poor interfacial regions.